Ethylene oxide is commercially produced by the epoxidation of ethylene over silver-containing catalysts at elevated temperature. Considerable research efforts have been devoted to providing catalysts that increase the efficiency, or selectivity, of the process to ethylene oxide.
The manufacture of ethylene oxide by the reaction of oxygen or oxygen-containing gases with ethylene in the presence of a silver catalyst is an old and developed art. For example, U.S. Pat. No. 2,040,782, patented May 12, 1936, describes the manufacture of ethylene oxide by the reaction of oxygen with ethylene in the presence of silver catalysts which contain a class of metal promoters. In Reissue U.S. Pat. No. 20,370, dated May 18, 1937, Leforte discloses that the formation of olefin oxides may be effected by causing olefins to combine directly with molecular oxygen in the presence of a silver catalyst. From that point on, the prior art has focused its efforts on improving the catalyst's efficiency in producing ethylene oxide.
Several terms are commonly used to describe some of the parameters of the catalytic system. For instance, "conversion" has been defined as the percentage of alkene or oxygen fed to the reactor which undergoes reaction. The "efficiency" or, as it is sometimes called, the "selectivity" of the overall process is an indication of the proportion, usually represented by a percentage, of the converted material or product which is alkene oxide. The commercial success of a reaction system depends in large measure on the efficiency of the system. Even a very small increase in efficiency will provide substantial cost benefits in large-scale operation. The product of the efficiency and the conversion is equal to the yield, or the percentage of the alkene fed that is converted into the corresponding oxide.
The "activity" of the catalyst is a term used to indicate the amount of alkene oxide contained in the outlet stream of the reactor relative to that in the inlet stream. Activity is generally expressed in terms of pounds of alkene oxide produced per cubic foot of catalyst per hour at specified reaction conditions and rate of feeds. The activity may also be stated in terms of the amount of alkylene oxide in the outlet stream or the difference between the alkylene oxide content of the inlet and outlet streams.
If the activity of a reaction system is low, then, all other things being equal, the commercial value of that system will be low. The lower the activity of a reaction system, the less product produced in a unit time for a given feed rate, reactor temperature, catalyst, surface area, et cetera. A low activity can render even a high efficiency process commercially impractical.
In some instances, activity is measured over a period of time in terms of the amount of alkylene oxide produced at a specified constant temperature. Alternatively, activity may be measured as a function of the temperature required to sustain production of a specified constant amount of alkylene oxide. The useful life of a reaction system is the length of time that reactants can be passed through the reaction system during which acceptable activity is observed.
Deactivation, as used herein, refers to a permanent loss of activity, i.e., a decrease in activity which cannot be recovered. As noted above, activity can be increased by raising the temperature, but the need to operate at a higher temperature to maintain a particular activity is representative of deactivation. Furthermore, catalysts tend to deactivate more rapidly when reaction is carried out at higher temperatures.
To be considered satisfactory, a catalyst must not only have a sufficient activity and the catalytic system provide an acceptable efficiency, but the catalyst must also demonstrate a minimum useful life or stability. When a catalyst is spent, typically the reactor must be shut down and partially dismantled to remove the spent catalyst. This results in losses in time and productivity. In addition, the catalyst must be replaced and the silver salvaged or, where possible, regenerated. Even when a catalyst is capable of regeneration in situ, generally production must be halted for some period of time. At best, replacement or regeneration of catalyst requires additional losses in time to treat the spent catalyst and, at worst, requires replacement of the catalyst with the associated costs.
Since even small improvements in activity, efficiency or useful life may have significance in large scale commercial production, such improvements have been the object of a great deal of research in the direct epoxidation of alkenes. The focus of attempts to improve performance, such as the activity and useful life of the catalyst and the efficiency of the system, has included such areas as feedstream additives or removal of components therefrom; methods of preparation of the catalyst; deposition or impregnation of a particular type or form of silver; composition, formation, physical properties and morphology of the support; additives deposited on or impregnated in the support; shape of support aggregates used in the reactor; and various types of reactors and bed designs, such as stationary and fluidized beds.
In general, the major thrusts in silver catalysts for alkylene epoxidation have been in the fields of promoter and modifier components for the catalytic system. Little attention has been given to the amount of silver contained in the catalysts. Indeed, the amount of silver in a catalyst has often been considered to be an economic trade-off. For instance, Kilty in U.S. Pat. No. 4,207,210, proposes, for ethylene oxide catalysts, a silver content range of 1 to 25 weight percent and states:
"The use of larger amounts of silver is not excluded but is generally economically unattractive." (Column 4, lines 15 and 16). PA0 "Greater amounts of silver are unduly expensive, while lesser amounts are not desirable, since useful life and activity of the catalyst are reduced." PA0 ". . . the use of a support material which is porous in nature and characterized by a limited range of pore diameters, the average pore diameter of which falls in a narrow range, can eliminate the heretofore universal need for halogenated inhibitors to temper or otherwise control the activity of the silver-containing catalysts employed in the controlled, partial oxidation of ethylene to ethylene oxide." (Column 2, lines 17 to 24). PA0 ". . . it would appear that catalyst centers exhibiting undesirable activity are minimized by the more homogeneous distribution of silver obtained by deposition thereof upon a porous support material wherein a substantial portion of the pores have diameters which fall within a limited range and wherein the average pore diameter falls within a narrow range." (Column 2, line 50 to 56). "The invention contemplates that the `average pore diameter` will be of a size such that neither too low [nor] too high a diffusion rate is encountered in practice." (Column 2, line 74 to column 3, line 1). PA0 "In order to increase the activity of the catalysts, the sufficient specific surface area of silver particles must be afforded. Therefore, the catalyst carriers are required to have enough specific surface area. However, oversized specific surface area will make the transfer of the reaction heat difficult, aggravate side reaction and decrease the selectivity of the catalysts. In order to offer the catalysts a high selectivity, an ideal pore structure which matches the surface of the catalyst is required so that suitable conditions for heat and mass transfer can be attained and side reaction can be suppressed. Since the reaction takes place under nearly diffusion-controlling conditions, searching for carriers with an optimum matching between pore structure and specific surface area has become an important subject in developing silver catalysts with a high selectivity." (page 2, lines 7 to 15) PA0 ". . . the coexistence of two ranges of porosity consisting of pores of different diameters was favorable to the selectivity of ethylene oxide." (Column 2, line 20 to 23).
Similarly, Armstrong in U.S. Pat. No. 4,342,677 states at column 4, lines 59 et seq.:
Armstrong broadly suggests that ethylene oxide catalysts can contain 5 to 50 weight percent silver.
Many prior workers have suggested the use of high silver loadings on ethylene oxide catalysts. For instance, Maxwell in U.S. Pat. No. 4,033,903 suggests the use of 1 to 35 weight percent silver; Hayden in U.S. Pat. No. 4,168,247 suggests 3 to 50 weight percent silver; Bhasin in U.S. Pat. No. 4,908,343 suggests 2 to 40 or more weight percent silver; Tamura in U.S. Pat. No. 4,645,754 suggests 5 to 30 weight percent silver; Sacken in U.S. Pat. No. 2,671,764 suggests 1 to 50 weight percent silver; Calcagno in U.S. Pat. No. 3,775,346 suggests 7 to 30 weight percent silver; and DeMaio in U.S. Pat. No. 3,664,970 suggests 5 to 30 weight percent silver. Yet, of these patents, only one, U.S. Pat. No. 4,908,343, provides a working example of a catalyst containing greater than 25 weight percent silver. Indeed, many workers propose a maximum silver content of 25 weight percent or less for ethylene oxide catalysts and exemplify catalysts containing only between 10 and 20 weight percent silver. Ethylene oxide catalysts manufactured on commercial bases at this time are believed to contain about 12 to 15 weight percent silver.
An insight as to why prior workers have generally tended to use silver contents in the 10 to 20 weight percent range may be perceived from a comparison of experiments using catalysts 43 and 52 of U.S. Pat. No. 4,168,247. The catalysts used in these experiments have the same carrier and promoter package but differ in silver content. Catalyst 43 contains 24 weight percent silver and catalyst 52 contains 8 weight percent silver. The oxygen conversions at 15 psia for both catalysts are identical (8 percent) and at 240 psia, the high silver content catalyst has an oxygen conversion of 3 percent while the lower silver content catalyst has an oxygen conversion of 2 percent. Under both pressure conditions, the lower silver content catalyst provides better selectivity than those exhibited using the higher silver content catalyst. This type of result would tend to confirm the observations of Kilty and Armstrong that little economic incentive exists for using high silver content catalysts.
One potential benefit of increased silver content is increased activity. Relative to efficiency gains, achieving increased activity can often readily be achieved by numerous techniques. Unfortunately, many of the techniques to increase catalyst activity, e.g., increased silver content, types and amounts of promoters and operating conditions including the presence and amounts of vapor phase modifiers such as ethylene dichloride, result in efficiency losses. Accordingly, catalysts are sought which can exhibit not only enhanced activity but also maintained or increased efficiency.
While little effort appears to be reported regarding high silver content ethylene oxide catalysts, work has been on-going in respect of carriers, or supports, for the catalysts. The carriers that have been employed are typically made of inorganic materials, generally of a mineral nature. In most cases, the preferred carrier is made of alpha-alumina, such as has been described in the patent literature: see for example, U.S. Pat. Nos. 2,294,383; 3,172,893; 3,332,887; 3,423,328 and 3,563,914.
The carriers which are employed for the manufacture of most, if not all, commercially employed ethylene oxide catalysts are produced by companies who do not produce such catalysts. As a rule, the methods of making such carriers are trade secrets of significant value to the carrier manufacturers. Consequently, the catalyst manufacturer cannot know how the carrier is made. The manufacture of a carrier for a successful ethylene oxide catalyst can involve a number of factors, such as the purity and other physical/chemical properties of raw materials used to make the carrier and the method by which the carrier is made. Description of carriers by many prior workers has thus been in terms of the chemical and/or physical properties of the catalyst.
DeMaio in U.S. Pat. No. 3,664,970 reflects the importance of physical properties of carriers in stating that:
DeMaio further postulates that:
DeMaio proposes that an average pore diameter of 4 to 10 microns is optimum without the need to employ a halogenated inhibitor. The patentee states that at least 90 percent of the pores have diameters in the range of 1 to 30 microns. No information is presented by the patentee on surface area of the carrier, and the reported porosities of the exemplified carriers ranged up to 56 percent.
Jin, et al., in European Patent Application 327,356 state:
Jin, et al., propose carrier for silver, epoxidation catalysts which have a specific surface area of 0.2 to 2 m.sup.2 /g, preferably 0.8 to 1.3 m.sup.2 /g; a total pore volume greater than 0.5 milliliters per gram, preferably 0.5 to 0.7 milliliters per gram; and pores of a pore radius of less than 30 microns comprise 75 to 90 percent of the total volume and those greater than 30 microns, between 25 and 10 percent of the total volume. In the examples, the predominant distribution of the pores in the carriers is in the 0.5 to 5 micron range. Jin, et al., state that the silver content is between 1 to 25 weight percent, but the examples use conventional silver loadings.
Hayden, et al., in U.S. Pat. No. 4,168,247 proposes the use of bimodal carrier for silver, epoxidation catalysts. The smaller pores are preferably to account for at least 70 percent of the total pore volume and have a mean pore diameter of 0.1 to 4 microns and the larger pores are to have a mean pore diameter of 25 to 500 microns. The apparent porosity is at least 20 percent, for example, 30 to 80 percent. The patentees propose that the amount of promoters present be in relation to the surface area of the support. Although silver contents ranging from 3 to 50% and more, preferably 3 to 30%, are suggested, no guidance is presented by the patentees as to the manner for effective utilization of the silver other than it should be in the form of discrete particles having equivalent diameters of less than 10000 Angstoms. As stated above, the higher silver content catalysts disclosed by Hayden, et al., provide no demonstrable benefit over lower silver content catalysts.
Cognion, et al., in U.S. Pat. No. 4,242,235 also disclose the use of bimodal carriers for silver, epoxidation catalysts. The patentees state:
The ranges are 1 to 5 microns and 60 to 200 microns. Each of these ranges preferably represents 35 to 65 percent of the total porosity. Other features of the support are said to be a surface area of less than 10 square meters per gram (m.sup.2 /g) advantageously, between 0.1 and 1 m.sup.2 /g and a porosity of up to 60 percent, preferably between 20 and 50 percent. The surface area of the carriers in the working examples is no more than 0.3 m.sup.2 /g and the maximum porosity is 0.34 cubic centimeters per gram.
Boehning, et al., in U.S. Pat. No. 4,829,043 disclose ethylene oxide catalysts in which carriers of certain physical properties are used to provide a silver density in the reactor of not less than 110 kilograms per cubic meter. The carrier has a surface area of 0.4 to 0.8 m.sup.2 /g and a pore volume of not less than 0.45 milliliter per gram. The patentees assert that the carrier is important to the activity of the catalysts and state that if the carrier is monomodal, the mean pore diameter is from 1 to 5 microns; and if bimodal, 50 percent of the total pore volume is of pores having a mean diameter of 10 to 40 microns and the smaller pores have a diameter of 0.5 to 2 microns.
Saffer in U.S. Pat. No. 3,207,700 discloses a composite support for silver, epoxidation catalysts. The outer margin is a porous material and a dense, substantially non-porous material forms the core. The outer margin has a porosity of 15 to 40 percent. The exemplified catalyst support has a surface area of the outer margin of less than 1 m.sup.2 /g, a porosity of 28 percent and about 80 percent of the pore volume is constituted by pores in the range of 1 to 3 microns. The composite carrier is said to provide substantially enhanced activity when formulated into a catalyst as compared to that of a catalyst formed from a homogeneous, high porosity support.
Numerous workers have disclosed broad ranges for carrier physical properties. Exemplary of such disclosures is Maxwell, in U.S. Pat. No. 4,033,903 who characterizes suitable carriers as having a surface area below 10 m.sup.2 /g and preferably below 2 m.sup.2 /g. The carriers are said to have an apparent porosity of greater than 20 percent. See also, for instance, Lauritzen, U.S. Pat. No. 4,761,394. Although broad ranges are presented, the actually exemplified carriers are often quite limited in scope. Lauritzen does exemplify one carrier, albeit a carrier containing only 70 to 75 weight percent alumina, having greater than 0.6 m.sup.2 /g surface area.
U.S. Pat. Nos. 4,368,144; 4,376,209; 4,389,338; 4,645,754; 4,769,358; 4,812,437 and 4,831,162 do, however, exemplify carriers for ethylene oxide catalysts which have higher surface areas, i.e., 0.7 m.sup.2 /g and above.
Although the foregoing documents indicate that work has been devoted to the development of carrier for silver, epoxidation catalysts, little, or no, guidance is given by these workers regarding the effect of carrier physical properties on the silver content of the catalysts. Often lower surface area carriers e.g., 0.3 to 0.6 m.sup.2 /g, are employed, within the field of catalysts which contain conventional amounts of silver.